The present invention relates to electrical penetrations which are utilized in sealing electrical conductors through a nuclear containment.
In nuclear power plants, a variety of electrical conductors must be sealingly brought through a containment wall which isolates the plant from the environment. The assembly for bringing the electrical conductors through the containment is termed a nuclear penetration. The penetration must be able to withstand high temperature and pressure, as well as high radioactivity levels. The seal assembly of the penetration must be able to withstand thermal cycling and heat aging in order to be acceptable for use in nuclear power plants.
A nuclear penetration design is set forth in copending application Ser. No. 432,375, filed Jan. 10, 1974, now U.S. Pat. No. 3,882,262 owned by the assignee of the present invention. This earlier penetration utilizes a generally tubular metal enclosure member which is adapted to be passed through a sealing flange which is sealingly mounted into a nozzle which extends through the containment. The electrical conductor is insulatingly passed through the tubular metal enclosure member, and an epoxy resin sealing material substantially fills the enclosure member to form a seal along and about the electrical conductor passing therethrough. A metallic seal shroud extends from the interior surface of the enclosure member, with the terminating end of the seal shroud spaced from the enclosure member. The epoxy resin sealing material fills the enclosure member and upon compressive setting forms a seal along and about the exterior surface of the seal shroud.
It is desired to increase the reliability of this seal design and to improve its thermal cycling characteristic.
It has been discovered that over an extended period with frequent extreme high temperature variations, the seal formed at the seal shroud may be damaged due to cracking of the cured epoxy resin. Under these extreme conditions the epoxy resin has cracked along a path extending from the free terminating end of the seal shroud outward through the cast epoxy body. When the seal structure is thermally cycled, a significant shear force is concentrated at the terminating end of the seal shroud. This end portion defines a boundary between the solid epoxy portion and the portion which is in compressive sealing engagement with the exterior surface of the seal shroud.
Various attempts to strengthen the seal structure with a modified epoxy composition or with varying filler and diluent amounts, did not significantly improve the thermal cycling characteristics of the seal structure.